Solder self-alignment methods

Abstract

A solder self-alignment process for aligning a semiconductor chip (13) with, and bonding the chip to, a substrate (12) is preformed in an atmosphere rich in gaseous formic acid, at least during the melting step. It is preferred that the formic acid atmosphere be maintained during the self-alignment step and the step of cooling and hardening the solder elements (23). With this feature, one can completely avoid the use of any solid or liquid fluxes and avoid the consequences of such use. Nonetheless, the molten solder elements (23) dependably bond to the bonding surfaces and vertically align themselves.

We claim: 
     
       1. A method for aligning a member with, and bonding the member to, a substrate, comprising the steps of: defining a first metal bonding pad on the member and a second metal bonding pad on the substrate, the first and second bonding pads being at predetermined corresponding locations;   coating at least one of the metal bonding pads with solder;   approximately registering the first bonding pad in close proximity to the second bonding pad;   melting the solder to cause the solder to gather and to make contact simultaneously with the first and second bonding pads;   using solder self-alignment to permit the surface tension of the molten solder to align the member with the substrate;   and cooling to harden the solder and to bond together the first and second bonding pads; wherein the invention comprises the step of:   exposing the solder and the bonding pads to an atmosphere rich in gaseous formic acid during at least the melting step.   
     
     
       2. The method of claim 1 wherein: the exposing step occurs also during the self-alignment step.   
     
     
       3. The method of claim 2 wherein: the exposing step occurs also during the cooling step.   
     
     
       4. The method of claim 3 further comprising the step of: bubbling a carrier gas through liquid formic acid to form gaseous formic acid and flowing the gaseous formic acid past the member and the substrate during the melting, self-alignment and cooling steps.   
     
     
       5. The method of claim 4 wherein: the carrier gas is substantially pure nitrogen.   
     
     
       6. The method of claim 1 wherein: the member is a semiconductor chip.   
     
     
       7. The method of claim 6 wherein: the semiconductor chip is a photonic device soldered in alignment with a mirror formed on the substrate; and further comprising the step of:   bonding to the substrate an optical fiber in alignment with said mirror.   
     
     
       8. The method of claim 1 wherein: the first metal bonding pad is one of a first plurality of bonding pads on the member, and the second metal bonding pad is one of a plurality of second metal bonding pads on the substrate, the locations of the second metal bonding pads matching the locations of the first metal bonding pads;   and solder self-alignment is used to solder bond each of the first bonding pads with a corresponding one of the second bonding pads.   
     
     
       9. The method of claim 8 wherein: the exposing step occurs also during the self-alignment step and during the cooling step.   
     
     
       10. The method of claim 9 wherein: the element is a photonic device;   and an optical device to be aligned with the photonic device is bonded to the substrate.   
     
     
       11. The method of claim 10 further comprising the step of: bubbling a carrier gas through liquid formic acid to form gaseous formic acid and flowing the gaseous formic acid past the photonic device and the substrate during the melting, self-alignment and cooling steps.   
     
     
       12. A method for aligning and bonding a semiconductor chip to a substrate comprising the steps of: defining a first set of metal bonding pads on the semiconductor chip and a second set of metal bonding pads on the substrate, the first and second sets having substantially identical configurations;   coating at least one set of metal bonding pads with solder;   locating the semiconductor chip above the substrate such that the first set of bonding pads is in approximate registration with the second set of bonding pads;   melting the solder to cause the solder to gather and to wet simultaneously the first and second sets of bonding pads;   permitting the surface tension of the molten solder to align the semiconductor chip with the substrate;   and cooling to harden the solder and to bond together the first and second sets of bonding pads; wherein the invention comprises the step of:   flowing vaporized formic acid past the semiconductor chip and substrate during the melting, self-alignment and cooling steps.   
     
     
       13. The method of claim 12 wherein: the semiconductor chip is a photonic device;   and the substrate includes bonded thereon an optical device to be aligned during the self-alignment step with the photonic device.   
     
     
       14. The method of claim 13 wherein: the photonic device is a photodetector;   and the optical device is an optical fiber.   
     
     
       15. The method of claim 14 further comprising the step of: bubbling a carrier gas through liquid formic acid to form vaporized formic acid and directing the vaporized formic acid to an enclosure containing the semiconductor chip and substrate.